The present invention relates to inductors, and more particularly, to an inductor used in integrated, circuits (ICs).
As the importance of communications services increases in modem information societies, high-speed, high-performance, highensity, high-reliability and low-cost electric devices are also required for a high quality, wide bandwidth, high data rate communication service, in addition to increasing the capability and speed of a channel. Thus, improving the structure and performance of radio frequency ICs (RFICs) and microwave monolithic ICs (MMICs) and hybrid ICs becomes the center of interest in the related art. In particular, improvement in the performance of an inductor which is a basic element used in designing voltage-controlled oscillators (VCOs), low noise amplifiers (INAs), narrow-ban impedance matching networks, high-performance linear filters, single-chip transceivers, multi-chip modules (MCMs) and low voltage/low power devices, which are key technologies in wireless communications, is very important. The performance of an inductor used for such RFICs and MMICs is evaluated according to Q-factor, inductance and self-resonant frequency, and typically an inductor having a high Q-factor is required in the technical field relating to the present invention [Seung-won Pack and Kwang-seok Seo, 1997, Air-Gap Stacked Spiral Inductor, IEEE Microwave and Guided Wave Letters, Vol. 7, No. 10, pp. 329-331].
Until now, horizontal spiral inductors have been used in RFICs and MMICs. However, the horizontal spiral inductor has disadvantages in that a relatively large area is occupied by the inductor with respect to the entire area of the RFIC or MMIC, and its Q-factor is low due to inevitable ohmic loss and dielectric loss. Also, due to an air-bridge process for connecting the center of the spiral inductor to a port, an additional photolithography and metallization processes are required in manufacturing the spiral inductor. Because of these problems, it is difficult to manufacture such a conventional spiral inductor. Also, after the manufacture of a mask used for the manufacture of the spiral inductor is completed, the inductance of an inductor manufactured thereby is fixed. Thus, in order to obtain different inductances, a separate mask must be manufactured for each conductor, thus increasing the manufacturing cost.
To solve the above problems, S. Chaki et al. suggests a method for increasing the Q-factor of an inductor by lowering resistance by increasing the thickness or width of a stripline formed by gold (Au) [S.Chaki, S. Aono, N. Andoh, Y. Sasaki, N. Tanini and O. Ishihara, 1995, Experimental Study on Spiral Induaors, IEEE MTTs Digest, pp. 735-756; M. Hirano, Y. Imai and K. Asai, 1991, xc2xc Miniaturized Passive Elements for GaAs MMICs, Proc. Of IEEE GaAs IC Symposium]. However, increasing the thickness of the stripline inevitably increases the unit cost of production, makes air-bridge processing for connecting the center of the inductor to a port difficult, increases the area occupied by the inductor in the RFIC or MMIC, and thus it is not suitable for low-cost mass production.
I. Woff and H. Kapusta disclose another method for solving the above problems by increasing inductance by increasing the length of a stripline. According to this method, however, the width of the stripline of a spiral inductor and the interval between the striplines become narrow, so that the resistance of the stripline increases and the Q-factor decreases. Thus, in order to increase inductance without a decrease in Q-factor, the area occupied by the spiral inductor must increase, thus increasing the manufacturing cost. Also, due to the loss of capacitance between a ground plane and the stripline and dielectric loss in a substrate, self-resonant frequency is liable to decrease [I. Wolf and H. Kapusta, 1987, Modeling of Circular Spiral Inductors for MMJCs, IEEE MITs Digest, pp. 123-126].
Also, Y. Seo et al. suggests using a multi-layered inductor as another method for increasing inductance [Y. Seo and V. Tripathi, 1995, Spiral Inductor in RFIC""s and MMICS, Proc. of Asia Pacific Microwave Conference, pp. 454-457; L. Zu and Y. Lu et al., 1996, High Q-factor Inductors Integrated on MCM Si Substrates, IEEE Trans. on Components, Packaging, and Manufacturing Tech. Part B, Vol. 19, No. 3, pp. 635-642]. Also, Y. Sun et al. reports an air-bridge inductor in which a wire is suspended from a dielectric substrate, as a method for decreasing conductance due to the capacitance and dielectric loss [Y. Sun, H. V. Zeiji, J. L. Tauritz and R. G. f. Baets, 1996, Suspended Membrane Inductors and Capacitors for Application in Silicon MMIC""s, IEEE Microwave and Millimeter-Wave Monolithic Circuits Symposium Digest, pp. 99-102; C. Y. Chii and G. M. Rebeiz, 1995, Planar Microwave and Millmeer-Wave Lumped Elements and Coupled-Line Filters Using Micro-Machining Techniques, IEEE Trans. on Microwave Theory and Tech., Vol. 43, No. 4, pp. 730-738]. However, these methods require a great expense for production, thus practical use thereof is difficult.
An object of the present invention is to improve the performance and economical problems of a conventional spiral inductor, and in particular, to improve a low quality (Q) factor and a low self-resonant frequency of the spiral inductor and difficulty in manufacturing the inductor;
Another object of the present invention is to provide a bonding wire inductor using a wire bonding technology which is widely used for integrated circuit packaging.
Another object of the present invention is to provide an on-chip solenoidal bonding wire inductor fabricated by a semiconductor manufacturing process.
Another object of the present invention is to provide a coupler or transformer using two bonding wire inductors arranged adjacent to each other.
Another object, of the present invention is to provide a surface mounted-type bonding wire inductor for a hybrid integrated circuits.
Another object of the present invention is to provide a vertical bonding wire inductor having, high Q-factor, high self-resonant frequency, and tunability which does not need an additional mask manufacturing process.
To achieve the above objects of the present invention, there are provided a bonding wire inductor having at least one pair of bonding pads which face each other, occupying a predetermined area on a substrate, wherein the facing bonding pads are connected by a bonding wire which is suspended from the substrate, thus forming a loop.
A pair of bonding pads and one bonding wire form a single loop bonding wire inductor. However, a bonding, wire inductor having 2, 3 or more loops can be manufactured as desired, and such a multiple bonding wire inductor falls within the scope of the present invention. In the case of a multi-loop bonding wire inductor, multiple bonding wires are arranged at a predetermined interval according to a pad pitch. As the pad pitch becomes narrow, the structure of the inductor becomes similar to that of a coil, increasing inductance. In the case of including two or more pairs of bonding pads, such bonding pads are arranged in two rows on a substrate. Also, a parallel connection between each of the bonding pads of one of the two rows and the bonding pad of the other row, the pads facing each other, and a diagonal connection between each of the bonding pads of one row and the corresponding bonding pad adjacent to the facing bonding pad of the other row, can be both connected by bonding wires as mentioned above, or the parallel or diagonal connection can be made by a metal strip which is in contact with the substrate.
Also, the bonding wire and bonding pads are ball-wedge bonded by automatic fine pitch ball bonding. Alternatively, the connection between the bonding wire and the bonding pads can be achieved by wedge-wedge bonding. In the case where the bonding wire is connected to the bonding pads by wedge-wedge bonding, the height of the loop formed by the bonding wire is lower than that in the case of using ball-wedge bonding. Also, the bonding wire and the bonding pads may be bonded by stitch bonding or ribbon bonding. In stitch bonding, the bonding length is short and the height of the loop formed by the bonding wire is low, compared to ball bonding (ball-wedge bonding), and thus stitch bonding has been widely used for packaging radio frequency circuits. In particular, the electrical performance of the bonding wire inductor according to the present invention is lowered by the resistance of the metal strip. Thus, by replacing the metal strip with a bonding wire, resistance of the metal strip can be decreased and unavoidable generation of the parasitic capacitance can be further decreased, thus increasing the Q-factor and resonant frequency.
In addition, the bonding wire may have a ribbon or round shape, or may have a rectangular section. The bonding wire may be formed of gold (Au), aluminum (Al), copper (Cu) or alloys thereof, and preferably, Au is used.
The metal strip which is in contact with the substrate may be formed of Au, Al, Cu or alloys thereof, and preferably, Au or alloys thereof is used.
According to the present invention, there is provided a method for manufacturing the bonding wire inductor, comprising the steps of forming at least one pair of bonding pads facing each other, on a substrate. Then, the facing bonding pads are connected with a bonding wire which is suspended from the substrate, thus forming a loop.
In a multi-loop bonding wire inductor having two or more pairs of bonding pads, such as the one mentioned above, some bonding pads can be connected by a metal strip, wherein the metal strip and the bonding pads can be formed by a lift-off method.
After a bonding process for connecting the bonding pads with bonding wires, the bonding wire inductor can be packaged with epoxy resin or can be fixed by hermetic packaging.
A semiconductor substrate on which the bonding wire inductor according to the present invention can be formed, may be a gallium arsenide (GaAs) substrate, a silicon substrate, an alumina substrate, a fluorine-resin substrate, an epoxy substrate, a ceramic substrate, a silicon-on-insulator (SOI) substrate, a lithium tantalate (LiTaO3) substrate, a lithium niobate (LiNbO3 substrate, a low temperature co-fired ceramic (LTCC) substrate, a quartz substrate, a glass substrate or a printed circuit board. Preferably, the silicon substrate or GaAs substrate is used. In general, in the case of using a silicon substrate, an insulating layer is formed on the silicon layer, and silicon oxide (SiO2), silicon nitride (Si3N4) or polyimide is used as an insulating material.
A bonding wire inductor can be achieved by fine pitch wire bonding equipment, which bonds highly elastic bonding wires having a diameter of 25-100 xcexcm. By using existing wire bonding technology used for RFICs and MMICs, which is automated and commercially available, the wire bonding can be implemented using bonding wires having a diameter of 25 xcexcm at a minimum pad pitch of 55 xcexcm.
Also, the thickness of the substrate may be approximately 100-625 xcexcm, and is preferably, about 100 xcexcm. The line width of the metal strip may be in the range of about 15-30 xcexcm, and the thickness thereof may be in the range of about 2-5 xcexcm. The Q-factor improves as the thickness of the metal strip increases. However, increasing the thickness of the metal strip requires additional expensive material, thereby increasing the unit cost for production. Preferably, the area of the bonding pad is in the range of about 50xc3x9750 xcexcm-90xc3x9790 xcexcm. In general, the performance of a device is improved with a decrease in the pad area, and thus the pad area can be reduced if required. The electrical characteristics of a bonding wire which forms a loop having a predetermined height, by connecting a pair of bonding pads, are improved as the diameter of the bonding wire increases. However, a bonding wire diameter of about 25 xcexcm is desirable. Also, preferably, in ball-wedge bonding, the height of the loop is in the range of about 70-1,000 xcexcm, and more preferably, about 350 xcexcm. Also, the bonding length between the bonding pad may be about 0.5 mm.
The area occupied by the bonding wire inductor according to the present invention is equal to that occupied by a spiral inductor. However, because bonding wires are suspended from a substrate, the effect of the magnetic field on other devices can be minimized, in addition to reducing the substrate loss (dielectric loss). As a result, a solenoid type inductor can be achieved, which is an ideal inductor suitable for high efficiency inductance. In a conventional spiral inductor, loss in a semiconductor substrate was a significant consideration in designing. Thus, in the aspect of substrate loss, the bonding wire inductor according to the present invention, which is suspended from the substrate, is highly effective in solving the problem of loss in silicon substrates.
Also, the bonding wire inductor according to the present invention can be applied in a coupler or transformer, by arranging two bonding wire inductors such that they are adjacent to each other. In arranging a plurality of bonding wire inductors in a voltage controlled oscillator (VCO) or other radio frequency devices, the bonding wire inductors can be arranged such that the directions of the magnetic fields are perpendicular to each other, in order to minimize magnetic interference.